Metabolism of bisphenol A in isolated rat hepatocytes and oestrogenic activity of a hydroxylated metabolite in MCF-7 human breast cancer cells

Abstract

1. The metabolites of bisphenol A (BPA; 2,2-bis(4-hydroxyphenyl)propane) in freshly isolated rat hepatocytes and the oestrogenic activities of BPA and its metabolites, particularly 3-hydroxybisphenol A (3-OH-BPA), in MCF-7 cells and competitive binding assays have been studied, respectively. 2. During a 2-h incubation, almost all of the BPA (0.25mM) added to the hepatocyte suspensions was rapidly converted to a major conjugate, monoglucuronide (approximately 75% of total metabolites), and two minor conjugates, which were tentatively identified as monosulphates of BPA and a hydroxylated intermediate, 3-OH-BPA, as determined by mass spectroscopy coupled with HPLC or GC/MS. On the other hand, free 3-OH-BPA was identified as a trace metabolite, whose level was approximately 1 or 2 μM at 1 h in hepatocyte suspensions treated with 0.25 or 0.5mM BPA, respectively. 3. In another experiment, 3-OH-BPA as well as BPA displaced competitively17β-oestradiol bound to the recombinant human oestrogen receptor α in a concentration dependent-manner: IC₅₀ of diethylstilbestrol, BPA and 3-OH-BPA were approximately 2.5 × 10⁻⁸, 10⁻⁵ and 5 × 10⁻⁸ M, respectively. Further, BPA and 3-OH-BPA at intermediate concentrations (10⁻⁷-10⁻⁶ M) caused proliferation of MCF-7 human breast cancer cells, whereas the effect of BPA was more potent than that of 3-OH-BPA. At higher concentrations, both BPA (> 10⁻⁴ M) and 3-OH-BPA (> 10⁻⁵ M) were cytotoxic. 4. Based on the proliferative potency in MCF-7 cells and the IC₅₀ for the competitive binding, the oestrogenic activity of 3-OH-BPA was less than that of BPA. These results indicate that BPA itself rather than its metabolite acts as a xeno-oestrogen and that 3-OHBPA is cytotoxic, possibly acting via reactive semiquinone and/or quinone metabolites, rather than a xeno-oestrogenic mechanism, in MCF-7 cells.